Merge pull request #112 from jonas-halbach/Lesson09Changes

added some additional explanation about maps and added an execise to …

09-ds-2-set-and-map/exercise.md

@@ -0,0 +1,20 @@
+---
+title: "Exercise - Video Store"
+nav_order: 0
+parent: "09 - Data Structure - Set & Map"
+nav_exclude: false
+---
+
+# Lesson 9: Data Structure - HashMap
+
+1. Create a new class Movie with title, director and year.
+2. Add a toString, equals and hashCode function to the class
+3. Create some instances of the Movie class in the Main method, add them to a hashmap (Hashmap<Movie, Integer> where the second parameter represents the number of available copies of this movie) and test if your implementation works as expected.
+4. Think about other properties of the movie class and adjust the methods mentioned in 2.
+5. Extract the code from 3 into a new class Movie-Store, where a user can buy a movie, which will reduce the number of movies in the stock.
+6. Build an text menu for your program where a user can buy a movie from the store.
+- The program shall ask a user for his actions:
+1. For list all available movies (Method signature public void printAvailableMovies())
+2. Buy a movie (Method signature public void buyMovie(String title, String director, int year))
+3. Quit the process
+The user can execute the program when choosing option 3

09-ds-2-set-and-map/index.md

@@ -89,6 +89,38 @@ public class Main {
 
 > As stated above, you need a `Map` when you are interested in keeping a dictionary or mapping of keys to values.
 
+### Useful methods of a Map
+
+- put -> adds a key and value to the map
+- get -> returns the value associated with the key
+- containsKey -> returns if the key exists in the map
+- remove -> removes the item with the given key from the map
+
+### Iterating over the elements of a Map
+
+Different options:
+
+#### By key:
+```java
+        for (String key: map.keySet()) {
+            int value = map.get(key);
+            System.out.println("The value associated to key " + key + " is " + value);
+        }
+```
+
+#### By EntrySet
+```java
+        for (Map.Entry<String, Integer> entry : map.entrySet()) {
+            System.out.println("The value associated to key " + entry.getKey() + " is " + entry.getValue());
+        }
+```
+
+## Using own objects as keys for a map or entry of a set
+
+If you want to use your own classes as keys for a map or element of a set you have to override the `equals` and the `hashCode` method of your new class.
+Please review [**this explanation**](https://www.geeksforgeeks.org/override-equalsobject-hashcode-method/){:target="_blank"} for more information.
+
+
 ## HashSet and HashMap
 
 - **Ordering is not important**
@@ -118,4 +150,4 @@ Define a class called `Ratings`, use a Map to store the ratings' information, an
 
 ## [Set & Map Assignment](https://classroom.github.com/a/dUh1YtIJ ){:target="_blank"}
 
-#### Follow the link, accept and download the assignment from GitHub Classroom
+#### Follow the link, accept and download the assignment from GitHub Classroom